i40e: Fix uninitialized variable mfs_max

The variable mfs_max is not initialized and is being compared to find
the maximum value. Fix this by initializing it to 0.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 90bc8e00 ("i40e: Add hardware configuration for software based DCB")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>